A fire alarm system typically includes one or more notification appliances that notify the public of fire or smoke. A Notification Appliance Circuit (NAC) connects the notification appliances to a fire alarm control panel. A primary power source (such as line power from an AC line) may supply power to the fire alarm control panel. The fire alarm system may also include a backup voltage source that supplies power to the fire alarm control panel. The backup voltage source is generally a low voltage source (such as a battery) that is used when the primary power source is unavailable. The NAC may provide power from the control panel to the notification appliances. The notification appliances draw a significant amount of current from the NAC and create a voltage drop across the NAC. The voltage drop may reduce the voltage supplied to the notification appliances at the end of the NAC (opposite the control panel) to a level that is below the voltage necessary to power the notification appliance.
During the design of the fire alarm system, a designer estimates whether all the notification appliances will have sufficient voltage to operate. To make this estimation, the designer predicts the voltage drop from the fire alarm panel to the last notification device. The voltage drop calculation is based on the length of the NAC, the resistance of the NAC, and the current draw of the appliances. The designer then subtracts the predicted voltage drop from the output voltage of the fire alarm panel and compares the result to the minimum operating voltage of the notification appliance. The NAC design is acceptable when the calculated voltage is above the minimum operating voltage of the notification appliance. However, the installed system may differ from the designed system. For example, the length of the NAC may differ due to practical considerations in the building, or alternate routings of the wires by the electrical installers. The actual voltage drop on a NAC in the installed system is frequently different than the calculated voltage drop. Therefore, it is important to confirm, after installation, that the NAC has sufficient voltage to operate the notification appliances.
Conventionally, it was difficult to test the voltage drop in an installed system. It was even more difficult to test the voltage drop at or near the lowest suitable voltage on the NAC. The lowest suitable voltage on the NAC is generally the voltage supplied from the control panel when the backup power source, for example, one or more batteries, are at the end of their rated life. The NAC voltage drop is difficult to determine at the lowest suitable voltage because the typical output voltage (supplied by the primary power source) of the control panel is significantly higher than the output voltage when the backup power source is supplying power. For example, the primary AC power source may supply a voltage that is many volts higher than the voltage supplied by the backup batteries, and thus the voltage output from the control panel is substantially different.
Because the notification appliances draw more current at low voltage than they do at higher voltages, an accurate measurement at the lowest voltage is difficult to obtain. If less current is drawn from the NAC, then the voltage drop across the NAC will also be reduced. Measuring the voltage at the control panel and then at the last notification appliance during higher voltage operation (supplied by the primary power source or the backup power source at the beginning of its rated life), will not give an accurate measurement of the voltage drop in the system during the lowest voltage operation (i.e. when the battery is at the end of its rated life).
In current systems the only way to measure the voltage drop on a NAC during the lowest voltage operation and verify that it is within its designed parameters, is to power the system from batteries for an extended period of time, until the batteries are near their rated end of life and then activate the notification appliances and measure the voltage drop on each NAC. This is generally not practical and is often not done because it is time consuming and potentially damaging to the batteries. Accordingly, a need exists for testing whether the NAC is capable of operating from batteries that is simple in design and operation.